1. Field of the Invention
This invention relates to electronic devices, including computer systems, and more particularly to electronic devices employing DC to DC power conversion.
2. Description of the Related Art
Personal computer systems in general and Intel/Microsoft compatible personal computer systems in particular have attained widespread acceptance. The term compatible is used to denote those computer systems employing microprocessor and chip set hardware supplied by Intel Corporation and operating system software supplied by Microsoft Corporation. These personal computer systems now provide computing power to many segments of today's modern society. A personal computer system can usually be defined as a desktop, floor-standing, or portable computer that includes a system unit having a processor with associated volatile and non-volatile memory, a display monitor, a keyboard, one or more floppy diskette drives, a mass storage device, an optional CD-ROM or DVD drive and an optional printer. One of the distinguishing characteristics of these systems is the use of a system board or motherboard to electrically connect these components together. These computer systems are information handling systems which are designed primarily to provide independent computing power to a single user, (or a relatively small group of users in the case of computer systems which serve as server systems.) Computer systems (and electronic devices generally) typically include a power supply which converts AC main power (120 volts in the United States, and 220 volts in many other countries) down to a smaller DC voltage useful for supplying the various components of the computer system. Different components of the computer system often have different DC voltage requirements. For example, the power rail which supplies an appropriate DC voltage to the processor of the computer system may have one voltage requirement. The L2 cache associated with the processor may have another voltage requirement while the system bus and peripherals may have still other voltage requirements. Computer systems typically include several DC to DC converters or regulators to down convert one DC voltage from the power supply to respective voltages used by the particular components of the computer system.
A conventional switched mode DC to DC buck regulator useful for this purpose is depicted in FIG. 1 as regulator 100. Regulator 100 includes a switching transistor 110 to which an input DC voltage V.sub.IN is provided. A free-wheeling diode 120 is coupled between the emitter of switching transistor 110 and ground. An inductor 130, with an inductance of 1 to 10 .mu.H for example, is coupled between load 150 and the node joining the emitter of transistor 110 and diode 120. The regulator produces an output voltage V.sub.OUT at node 160. A switch control circuit 180 (e.g., a pulse width modulator integrated circuit, or a voltage controlled oscillator) is coupled between output node 160 and the input or base of switching transistor 110. Switch control circuit 180 senses the output voltage V.sub.OUT as the load 150 varies and appropriately adjusts the switching frequency of transistor 110 by, for example, adjusting the pulse width of the control signal applied to switching transistor 110 to dynamically regulate the output voltage V.sub.OUT to the desired value.
During normal operation, the load presented to a DC to DC regulator can have minor to major current fluctuations. For example, the DC to DC regulator which supplies power to a processor will experience high frequency load fluctuations at the processor switching rate which is in the MHz range. The average load current can be substantially constant with minor fluctuations or can change dramatically due to a change in processor state condition such as transitioning from a sleep state to a fully active state.
Thus, the DC to DC regulator must be able to deal with both major and minor fluctuations in load current. The system bus and other buses of the system experience significant changes in operating current as well. Moreover, other I/O devices in the system, such as hard drives, floppy drives, CD ROMs, DVDs also present varying current requirements. It is important that a DC to DC regulator be able to provide relatively constant DC output voltage as the load dynamically changes.
Conventional DC to DC buck regulator 100 responds to load changes in the following manner. DC to DC buck regulator 100 includes one or more transition control or bypass capacitors 170. The capacitor 170 is coupled between the output rail and ground as shown in FIG. 1. Typically, capacitor 170 is physically located adjacent to load 150. Capacitor 170 functions as a high frequency bypass capacitor which controls the noise with respect to ground on the rail and at node 160, such noise being due to the switching transitions of the processor or other load 150 presented to the regulator. Capacitor 170 is typically a low ESR (equivalent series resistance) and/or a low ESL (equivalent series inductance) device. For this reason, when charged, the capacitor is capable of maintaining the voltage at load 150 by becoming a source of current into the load when high frequency load transitions occur. This can occur because of the very low parasitic series resistive and inductive properties of this type of capacitor as compared to the output capacitor 140 discussed below. Capacitor 170 is typically a relatively small capacitor such as 1 to 22.mu.F tantalum or ceramic capacitor.
To address major load fluctuations, such as when the load changes dramatically from, for example, 5A to 35A, regulator 100 employs one or more output capacitors 140 coupled between the output rail and ground as shown. Capacitor 140 typically is a relatively large capacitor such as, for example, an 820 .mu.F to 3900.mu.F high performance aluminum electrolytic capacitor. Capacitor 140 (or multiple capacitors 140) functions as a bulk capacitor or output capacitor for the regulator. Together with inductor 130, capacitor 140 forms an LC filter or tank circuit. When bypass capacitor 170 experiences a transition at the processor rate, it dumps current into the load to control the transition. At the same time, the output capacitor 140 starts to replenish the energy of capacitor 170. This is one of the functions of the output capacitor 140. It is noted that for minor load variations as well, capacitor 140 still replenishes capacitor 170 except to a lesser magnitude according to the lesser needs of 170 during a minor load fluctuation.
To summarize, capacitor 170 provides a substantial energy source during each processor cycle (or other load cycle) independent of average processor load. In between each processor clock cycle there is energy replacement or replenishment from capacitor 140 to capacitor 170 at a somewhat lower transfer rate than the processor consumption rate at processor clock transitions. That replenishment rate is defined mostly by parasitic inductances within capacitor 140 or physical implementation limitations such as printed circuit board (PCB) impedances or connector impedances between 140 and 170, or other parasitic losses. During a very large step load change the amount of energy replenishment from 140 to 170 will be proportional to the magnitude of the step load change.
As load power requirements, and particularly transient current requirements increase, the power loss and noise associated with both the regulator and the parasitic effects occurring in the wires or circuit board traces connecting the regulator to the load become more undesirable. One attempt to reduce the effect of parasitic losses (e.g. resistive and inductive losses) is to locate the regulator (whether a regulator circuit located directly on the system board or a voltage regulator module (VRM) which is a separate circuit board containing all regulator components and connected to the system board in a socket or connector) close to the load, such as a processor. By reducing the length of the conductive path between the output of the regulator and the load, parasitic effects are reduced.
FIG. 2A illustrates the connector layout for a typical multiple-processor system board 200. Processor slot connectors 210, 212, 214, and 216 each can connect one processor card unit to system board 200. VRM connector 220 connects a VRM for supplying regulated power to the processor core of a processor inserted into slot connector 210. Similarly, VRM connectors 222, 224, and 226 are associated with connectors 212, 214, and 216, respectively. VRM connector 221 connects a VRM for supplying regulated power to the level two (L2) cache memory of the processors in slot connectors 210 and 212. VRM connector 225 serves a similar function for processors in slot connector 214 and 216. Connector 230 is for a clustering module so that multiple system boards can be linked together, and VRM connectors 232 and 234 provide for the VRMs needed by the clustering module. System board 200 includes a variety of other features such as I/O bus connectors 240 and 245, memory connectors 250 and 255, and socket 260 for a system board support chip. FIG. 2B shows system board 200 with four processor cartridges 270 installed in the connectors 210, 212, 214, and 216. As illustrated by FIG. 2B, board space is at a premium, particularly because of the size of the processor cartridges and the number of VRMs that must be accommodated.
As illustrated in FIGS. 2A-2B, there are problems associated with locating VRMs or regulator circuitry close to a load device. First, in many computer systems, allocation of system board space for components is tightly controlled. In many computer systems, particularly multiple-processor systems, multiple regulators are required, thereby compounding the board space allocation problem. Moreover, with closely spaced electronic components, cooling the components becomes more difficult, and components that extend up from the system board and hang over other portions of the system board (e.g., slot-based processor cards with attached heat-sinks and fans) often come into contact with other system components. Secondly, if regulators such as VRMs are used, it is difficult to get some regulator components sufficiently close to the load to reduce parasitic losses.
One solution for the problem of allocating system board space is to relocate the regulator to areas of the system board where there is additional space, or to relocate the regulator to a separate circuit board. Clearly, this is counterproductive to efforts to reduce parasitic losses by locating the regulator close to the load. Additionally, if the regulator must be located further away from the load, extra steps need to be taken to compensate for the loss in performance, including, for example adding reservoir capacitors close to the load, and requiring more strict performance tolerances of the regulator, both of which add complexity and cost to any design.
Accordingly, it is desirable to have a DC to DC regulator that uses a relatively small amount of system board space, yet is located sufficiently close to a load so as to reduce power loss and noise associated with the known parasitic effects.